The invention relates to a method to drive a machine having at least one component carrying out a periodic motion and an electric drive motor, and to a drive means for such a machine.
In the present invention, the expression xe2x80x9cmachine containing at least one component carrying out a periodic motionxe2x80x9d connotes that this component does not carry out a continuous, constant and illustratively rotary motion. For instance a weaving machine comprises several such components carrying out periodic motions. Such a component for instance is the batten which is pivoted to-and-fro at given times. The shed forming means are another such component which is moved up and down at specific times. Gripper weaving machines include grippers and their drive elements as components that reciprocate at given times. Where several components carrying out periodic motions are present, a resultant composite period will be present. As regards to weaving machines, this period is determined by the number of weaving cycles in a weaving pattern. The interlacing of filling and warp threads repeats accordingly in a repeat pattern.
If now such a machine, for instance a weaving machine, is driven by an electric asynchronous motor fed from a power line, then this motor will be severely loaded. As a result it will operate at lower efficiency. This lower efficiency is explained foremost by the angular speed of the drive shaft of such a weaving machine and hence the strongly varying angular speed of the drive motor. Relative to nominal angular speed, the higher and/or lower actual angular speed causes the drive motor to draw large currents. These large currents entail high energy losses due to the drive motor""s heat dissipation and only ineffectively contribute to that torque generated by the motor which is required to keep the weaving machine""s angular speed constant.
It is known to mount a flywheel on such a weaving machine in order to reduce such drawbacks. This flywheel substantially improves the efficiency of an asynchronous motor operating as the drive motor. However large torques are exchanged between the flywheel and the weaving machine, the latter being much stressed as a result and undergoing wear thereby. Not only high torques, but also large torque differentials, furthermore, are disadvantageous.
When the flywheel is linked by a clutch to the weaving machine, the flywheel""s kinetic energy is available to rapidly start this weaving machine. However this design entails the drawback that the link means must absorb large torques. Another drawback is that after starting, the weaving machine reaches fairly quickly an angular speed which is only a percentage of the working angular speed, for instance 80%. However, on account of the flywheel""s moment of inertia, thereafter the working angular speed is reached relatively late. Furthermore the percentage of the working angular speed depends on ambient factors, for instance the temperature of the weaving machine, the amplitude of the line voltage, the humidity in the weaving room, the shutdown time of this weaving machine etc. This condition is especially disadvantageous in weaving machines because of the degradation in quality of the woven fabric.
It is further known in the weaving machine art to increase the angular speed of the flywheel in relation to the working angular speed before starting the weaving machine in order to then rapidly reach a higher percentage of the working angular speed. However the angular speed that is attained remains dependent on the above cited ambient factors and again there is loss of quality of the woven fabric.
The objective of the invention is to create a drive means for the machine of the initially cited kind and to make this drive means efficient, such that it will incur only minor energy losses.
This problem is solved in that the drive motor is controlled in such a way that the torque applied by the drive motor to the machine will be predetermined.
By controlling the applied torque, the drive motor need not operate at higher force or power against the machine""s moment of inertia, that is, it will not act as a brake when the moment of inertia acts as a bias towards increasing the machine""s speed, nor shall it react by an increased torque when the moment of inertial tends to reduce the speed. Accordingly the drive motor is controlled in such manner that it follows the machine""s angular speed resulting from this machine""s moment of inertia. When the machine""s moment of inertia leads to a drop in speed, the drive motor goes along, just as it goes along when the moment of inertia leads to an increase in the speed of the machine. While the speed fluctuations of the machine are somewhat increased thereby, this aspect of the invention as a rule shall not operationally interfere. Especially in weaving machines, larger speed fluctuations will not interfere over one period or over one weaving cycle.
The invention offers the advantage that the torque applied by the drive motor can be preset and is selected in such manner that the energy losses in the drive motor shall be limited and furthermore, the loading and/or the loading differential in the machine""s drive shaft and/or in the motor""s shaft is also reduced. The drive-motor torque preferably is predetermined in such a way that this drive motor always can transfer energy to the machine. As a result, a drive motor capable of high power over a long time need not be used, instead one may use a comparatively economical small drive motor which moreover operates at advantageous efficiency. It is assumed in this respect that the drive motor need not be unduly loaded to attain an approximately constant angular machine speed. A variation in machine angular speed is allowable and there is no need for the drive motor to provide high power at given times in order to maintain a constant angular speed. The attempt to keep the angular speed constant leads to the drawback that it requires a costly, high-power drive motor in order to apply transient high power. Such high power would be much larger than the average power required of the drive motor and this imbalance would be manifested in energy losses, i.e. heat dissipation, and hence resulting in unsatisfactory drive-motor efficiency.
In another design of the invention, drive-motor control data are retrievably stored to operate the drive motor at any angular speed by means of a plurality of different torques in such manner that the angular position of the drive motor is detected and in that the value of the torque in the memory corresponding to the detected speed is set as the torque to be applied. One simple solution is that the applied torque shall be constant.
In another design of the invention, the torque to be applied by the drive motor is stored as a function of this drive motor""s angular position, the instantaneous angular position of the drive motor is detected and the torque value relating to the instantaneous angular position is read in the memory and is then predetermined as the torque to be applied. In this manner the function of the applied torque is matched to the moment-of-inertia function.
Especially with regard to weaving machines, the torques to be applied shall be advantageously stored in relation to the drive motor""s angular positions. In this case the function of that moment of inertia can be selected for which the weaving machine shall weave optimally for a given material.
In another design of the invention, the angular-speed function of the drive motor of which the torque is being controlled is measured and stored in relation to the drive motor""s angular positions and the machine is started by a start circuit by means of which the drive motor is regulated by an angular speed control at an angular speed which is stored in relation to the instantaneous angular position of the drive motor. This configuration offers the advantage that the drive motor can be started in such a way that the machine runs at an angular speed function already a short time after starting which already corresponds to the speed function that the machine would obey if it had not been stopped. As a result the machine operates already shortly after starting at good efficiency. Because controlling the speed of the drive motor causes torques which substantially are equal to the angular speeds the motor would be running at if the torques were controlled, the drive motor again operates efficiently. This starting circuit is especially advantageous for a weaving machine. Because in practice the speed function shortly after starting corresponds to that in effect before the weaving machine was stopped, this weaving machine practically will be operated at a speed the same as before the stoppage at the beatup of the first inserted filling. This feature is independent of ambient effects. In this manner the fillings can be woven the same way after the start of the weaving machine as before stopping it. Consequently the quality-degrading starting marks in the fabric can be substantially avoided. Moreover this latter advantage also is enjoyed when there are changes in weaving-machine temperature or other ambient effects at the time of starting relative to the previous temperatures and ambient effects.
In a further embodiment of the invention, the machine is shut off by a shutdown circuit decelerating the drive motor to angular speeds associated to predetermined angular position, the drive motor coming to rest in a predetermined angular position. The latter feature is especially advantageous in weaving machines, particularly when a filling rupture or the like must be remedied.